Conventional structures (e.g., metallic, cement, wood, stone, brick, etc.) are being replaced with composite structures. Composites are typically made from two components: highly oriented, high strength/high modulus fibers embedded in a high performance matrix resin. Composite structures offer several advantages over conventional structures including excellent overall strength and stiffness to weight ratios, superior environmental resistance, better fatigue resistance, low thermal expansion characteristics, excellent fire resistance, and electrically and magnetically tailorable properties. To date, the acquisition costs of most composites have remained unacceptably high and have in effect prevented composites from replacing a much greater percentage of the conventional structure materials market.
Prices for composites have remained unacceptably high despite recent significant reductions in the costs of the raw materials that are used to make composites. Excessively high production associated costs are acting as a price barrier preventing composite structures from achieving a competitive advantage over most conventional structures. Production costs are driven by three primary factors: the costs associated with "laying down" the highly oriented fibers, the costs associated with effecting an adequate cure of the matrix resin, and the costs associated with the fabrication of composite worthy tools. Further aggravating the cost situation is the environmental problem involved in dealing with the disposal of the waste solvents given off in the cure of typical resin systems.
The first cost driver (applying the fibers onto a tool) is a problem that has been attacked with a variety of strategies. Among the alternatives that are currently being used are: hand layup, robotic "tape" laying machines, filament winding/braiding machines, and pultrusion machines.
Hand layup obviously requires high labor costs and results in low production rates.
Robotic "tape" laying machines eliminate much of the labor expense but utilize very expensive computer hardware/software. Relatively low production rates are obtained using robotic tape laying machines primarily caused by the requirement that the head of the tape spindle applies only one ply per pass.
In a filament winding/braiding process, a band of continuous rovings or monofilaments is wrapped around a mandrel and cured to produce axisymmetric hollow parts. Among the applications of filament winding/braiding are automotive drive shafts, helicopter blades, oxygen tanks, pipelines, spherical pressure vessels, conical rocket motor cases, and large underground gasoline storage tanks. United States patents describing various applications of the filament winding/braiding process are U.S. Pat. Nos. 4,892,764; 3,992,426; 4,515,737; 4,230,766; 4,479,984; and Re. U.S. Pat. No. 27,973. Filament winding/braiding machines are able to apply large amounts of material at high rates of speed with great economy. However, one is left with the problem of removing the volatile solvents, and the mandrel presents a problem of disposition after the article has been completed. In the vast majority of cases it is undesirable to leave the mandrel encased inside the composite part. To remove a mandrel often requires that the mandrel either (1) be capable of disassembly; (2) be made of a material that can be washed/flushed or melted out of the composite cavity; or (3) be inflatable thereby permitting it to be decompressed and then evacuated through an appropriate aperture in the composite skin.
Pultrusion machines essentially pull resin drenched fibril rovings through a female die while at the same time introducing heat to effect a cure. United States patents describing various forms of pultrusion machines are U.S. Pat. Nos. 4,477,702; 4,012,267; and U.S. No. 4,892,764. Generally, the heat in a pultrusion procedure must travel from the outside of the composite part at a finite rate. In such case it becomes necessary for the machine's cure station to be long enough to allow sufficient time for heat penetration or in the alternative it is necessary for the composite part to move through a shorter cure station at a reduced velocity. In the case of a radio frequency cured pultrusion machine, the cure is still thermal. In order for the polymers to form correctly the article being cured must not be physically manipulated during the cure. To allow extensive flexure would deleteriously affect the polymerization process. Thus in all thermal cases, the cure die must be long enough to support the composite part during the cure. This necessitates parts to be axial symmetric. The practice of making a long cure die and/or using slow flow rates increases costs. The relatively slow rate of heat transfer places an effective limit on the maximum wall thickness that can economically be produced in a pultruded part. The inability to achieve sufficiently thick wall thicknesses can eliminate the possibility of using the pultrusion technique for a wide variety of structural applications.
The second cost driver is the cost of effecting the cure of the resin matrix. In all the above systems it is necessary to introduce the matrix resin into the fibers. This can be done either before, during, or after the fibers have been placed on a tool or mandrel. However, almost all resin systems in use today are made of solids suspended in a variety of solvents. Depending on the method of fiber lay up, there may also be trapped foreign fluid. These solvents and foreign fluids must be removed from the composite part to reduce a detrimental condition referred to as porosity. In the majority of cases this requires the use of considerable consolidation pressure to squeeze the volatiles out of the curing part. The consolidation pressure is usually applied either by bagging the part under vacuum or using a wrapping of plastic shrink wrap. The process of bagging a part is time consuming and requires a significant expenditure of materials and hand labor. The solvents that are released during the cure pose an environmental risk as well as a risk to those who must handle the raw materials.
The third cost driver is the cost of making composite-worthy tooling. Composite tooling is fundamentally different from most metallic tooling inasmuch as composite tooling is specific to the part being fabricated whereas metallic tooling tends to be more universal. Composite tooling tends to be either a mandrel (see above) or a mold formed in the shape of the desired part. Routinely composite tooling must be capable of withstanding elevated temperatures and pressures. This tends to force composite tooling to be structurally robust and therefore expensive. Pultrusion dies are also expensive and are also very part specific. Metallic structural tooling design has had longer to evolve, and this has led to machining techniques that are far less part specific and more universal. It is a disadvantage of the production techniques presently practiced in the composite industry that they suffer from the lack of flexibility, adaptability and universality enjoyed by metallic machine processes.
It is an object of the present invention to provide a method which substantially reduces the several cost driving problems referred to above.
It is another object of the invention to provide a specialized procedure which allows radiant energy to be used to achieve an exceptionally fast cure of the fiber/matrix combination thereby permitting much greater flexibility in what types of parts can be fabricated at significantly reduced costs, or greatly reduced emission of solvents into the environment, or both.
It is a further object of the invention to provide a method which permits the use of the high speed and economy of filament winding/braiding machines while at the same time eliminating the need for a mandrel and the problems associated with disposing of it.
It is a still further object of the invention to eliminate the need for a mandrel in hand lay up and other methods.
It is a still further object of the invention to provide a method which eliminates the need for long cure dies in the pultrusion process, thereby speeding up the rate of production and enabling the manufacture of pultruded parts having greatly increased wall thicknesses and a great variety of non-axial parts.
Other objects and advantages of the invention will become apparent as the specification proceeds.